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The Bleaching of Mentawai Island Corals During the 1997-98 El Niņo
Volume 12, Number 15: 15 April 2009

In late 1997 -- during the midst of a strong El Niņo that pushed global mean temperature to a level of warmth that has not subsequently been eclipsed -- close to 100% of the coral and fish in the reef ecosystem of the Mentawai Islands (located southwest of Sumatra, Indonesia, in the equatorial eastern Indian Ocean) were killed. However, it was not the debilitating global warmth that led to their demise, for water temperatures around the islands at that time were significantly colder than usual, due to enhanced southeasterly trade winds that drove strong upwelling along the southwest coast of Sumatra, causing sea surface temperatures (SSTs) in the Mentawai reefs to drop by about 4°C.

Noting that the cool SSTs were due to a recurrent phenomenon termed the Indian Ocean Dipole (IOD), Abram et al. (2003) decided to explore the unusual mix of observations in more detail. Working with the skeletons of modern and fossil Porites corals from the Mentawai reefs, they reconstructed the magnitudes of recent and prehistoric IOD events using strontium/calcium ratios (Sr/Ca) as proxies for SST, along with oxygen isotope ratios (δ18O), which reflect SST and salinity, while they used stable carbon isotope ratios (δ13C) and manganese (Mn) and rare earth element (REE) concentrations as proxies for biological activity. These efforts revealed the occurrence of seven prior upwelling events that produced SST reductions of 2.8 to 5.8°C; but they say that "in all cases there is no x-ray evidence for discontinuous coral growth during upwelling," such as what occurred at the peak of the 1997-98 IOD, which pretty much eliminated the drop in temperature as a direct cause of the deadly bleaching.

Another thing about the 1997-98 coral die-off that intrigued the five researchers were eye-witness reports of a concurrent large phytoplankton bloom or "massive red tide," as they describe it, that "extended several hundred kilometers along the island chain during December 1997-January 1998." In commenting on these observations, Abram et al. note that red tides can kill marine organisms by asphyxiation caused by oxidation of abundant dead biomass; and they report that "strong enrichments in coral Mn and REEs in late 1997 provide evidence for low oxygen levels in the water column at the time of the Mentawai reef mortality and suggest that asphyxiation was the likely cause of death for the coral and fish." And since there was no δ13C evidence for a red tide associated with any of the paleo-upwellings they had identified, and there was an absence of growth unconformities in 48 Porites cores dating back 7000 years, the question of what caused the unique 1997-98 coral mortality thus boiled down to: What caused the unprecedented red tide?

In solving the mystery, the researchers began by reminding us that "primary productivity in coastal upwelling systems is generally iron (Fe) limited, with external inputs of Fe required to balance the up-welled sources of macronutrients (N and P)." Calculating that the flux of Fe required to sustain the unique Mentawai red tide was far above the average deposition rate of bio-available aeolian Fe in the area, they concluded that "the additional Fe required to support the Mentawai red tide was provided by atmospheric fallout from the 1997 Indonesian wildfires," which they describe as being "the worst wildfires in the recorded history of southeast Asia." In addition, they report that "anomalous equatorial easterlies resulted in persistent transport of smoke over the Mentawai Islands from September to December 1997," with "the highest density of smoke consistently located over the Mentawai area."

This scenario fits all the constraints of the researchers' many observations and provides a reasonable explanation for what occurred in the Mentawai reefs during the 1997-98 IOD. Hence, they concluded their paper with the warning that "widespread tropical wildfire is a recent phenomenon, the magnitude and frequency of which are increasing as population rises and terrestrial biomass continues to be disrupted," and by further stating that "reefs are likely to become increasingly susceptible to large algal blooms triggered by episodic nutrient enrichment from wildfires," which phenomenon, in their words, "may pose a new threat to coastal marine ecosystems that could escalate into the 21st century."

Clearly, there is nothing we can do to reduce the occurrence of periodic El Niņos, which periodically raise havoc with many of earth's coral reefs; but there is something we can do to help prevent massive wildfires, and to reduce many of the other deleterious effects of our day-to-day activities that negatively impact reef environments, of which there are more than enough to engage all of the time and resources we might wish to devote to that purpose. And this is what we must do, if we are truly serious about saving earth's corals for future generations.

Sherwood, Keith and Craig Idso

Abram, N.J., Gagan, M.K., McCulloch, M.T., Chappell, J. and Hantoro, W.S. 2003. Coral reef death during the 1997 Indian Ocean Dipole linked to Indonesian wildfires. Science 301: 952-955.